Scheme system and method for power saving in vectored communications

ABSTRACT

A transmission scheme for a plurality of transceiver pairs, the transmission scheme including a partition of at least a downlink transmission portion of a data transmission frame into a plurality of precoding intervals, each precoding interval being associated with a respective active group of transceiver pairs, each active group of transceiver pairs including a plurality of transceiver pairs, each precoding interval terminating no earlier than the end of a respective downlink data transmission period associated with at least one of the transceiver pairs in the respective transceiver active group, wherein, a different respective precoder is associated with each the active groups and wherein, at least the transmitting ports of the first transceivers, which are not members of an active group, turn off.

FIELD OF THE DISCLOSED TECHNIQUE

The disclosed technique relates to time division duplexing (TDD) inmodem communications, in general, and to implementing vectoringtechnology, in particular.

BACKGROUND OF THE DISCLOSED TECHNIQUE

The “last mile” is a phrase in telecommunications, cable television andinternet industries relating to the connection of retail customers(e.g., homes or offices) to the pertinent network (e.g., the telephonenetwork or the internet). The “last mile” connections, generally consistof copper twisted pairs, typically exhibit a bandwidth “bottleneck”limiting the rate of data delivery to the customers since twisted pairswhere originally deployed to support voice signals, and not tocommunicate high bit rates typical to broadband Internet access.Furthermore, “last mile” connections are expensive to upgrade (e.g., tooptical fibers) due to the large number of such connections (i.e.,relative to the number of connections between exchanges or routers).

Reference is now made to FIG. 1, which is a schematic illustration of atypical “last mile” connection which is known in the art. Such a typical“last mile” connection includes a building 10 a distribution point (DP)20 and a central office 24. Building 10 includes, for example, eightapartments 12 ₁-12 ₈. Each of apartments 12 ₁-12 ₈ includes, forexample, a respective one of computers 14 ₁-14 ₈ coupled with arespective one of modems 16 ₁-16 ₈ either directly or via a router orhub (not shown). Each one of modems 16 ₁-16 ₈ is coupled withdistribution point 20 via a respective one of line connections 18 ₁-18 ₈also known as “drops”. Each one of line connections 18 ₁-18 ₈ is, forexample, a twisted pair of wires. Each one of line connections 18 ₁-18 ₈may further be, for example, a coaxial cable. Line connections 18 ₁-18 ₈are grouped together within a binder 22. Distribution point 20 alongwith line connections 18 ₁-18 ₈ and computers 14 ₁-14 ₈ constitute acommunication system. Distribution point 20 is coupled with Centraloffice 24 via a communication channel 26 (e.g., optical fiber, cable,wireless channel). The distance between building 10 and distributionpoint 20 is up to the order of hundreds of meters and typically up to200 meters. The distance between distribution point 20 and centraloffice 24 is up to the order of several kilometers.

It is noted that computers 14 ₁-14 ₈ are brought herein as an exampleonly. Other devices require communication services (e.g., smart TV's,smartphones, IP phones, routers) may be coupled with the respective oneof modems 16 ₁-16 ₈. Furthermore, building 10 may include offices ratherthan apartments. Additionally, the number of apartments or offices inbuilding 10 may be different than eight (e.g. four, sixteen).Additionally, the distribution point may be connected to a plurality ofprivate homes.

Data transmission includes downstream transmission of data from the DPtoward the CPE also referred to as downlink (DL). Data transmission alsoincludes upstream transmission of data from the CPE toward the DP alsoreferred to as uplink (UL). Furthermore, data transmission is dividedinto data frames, where each frame includes a plurality of time-slotseach for transmitting a data symbols (i.e., a combination of bits, whichis encoded and modulated to create the data symbol exhibiting theduration of each time-slot). Nevertheless, the terms ‘time-slot’ and‘symbol’ are used herein interchangeably. In each frame, a portion ofthe symbols may be designated for downlink transmission and a portion ofthe symbols may be designated for uplink transmission. Frames mayfurther be grouped in super-frames, where each super-frame includes aplurality (e.g., on the order of tens) of frames. Reference is now madeto FIG. 2 which is a schematic of a super-frame, generally referenced50, which is known in the art. Super-frame 50 includes a plurality offrames. The duration of super-frame 50 may be on the order of severalmilliseconds (ms) and typically 6 ms and each frame typically includesbetween 20 symbols and 40 symbols. Each frame, for example, frame 52,which corresponds to the second frame of super-frame 50, includes aplurality of time-slots, such as time-slot 54 for transmission of datasymbols.

“G.fast” technology attempts to increase the data rate between thedistribution point and the Customer Premise Equipment (CPE—such asmodems, routers, hubs, computers, Smart TV's and the like) to the orderof one Giga bits per second (i.e., 1 Gbps). Typically, the bandwidth ofeach twisted pair is between 100-200 Megahertz (MHz) and the number oftwisted pairs per binder is between eight and sixteen. As a result ofthe high frequencies employed, a high degree of cross-talk interferenceexists between the different twisted pairs in the binder. In essence,due to the high level of cross-talk, the coupling between thedistribution point and different CPE's may be considered as a multipleaccess problem where a plurality of devices are coupled with theplurality of CPE's. Such a coupling or channel may be described in amatrix form where the entries in the matrix represent the differentcoupling factors. Cancelling cross-talk interference is also referred toas “vectoring”. Vectoring means the use of one or both of precoding inthe downlink direction and cross-talk cancellation in the uplinkdirection. In general, power consumption is of utmost importance forsystem implementations designed to be installed in the distributionpoint.

In general, two primary parameters are associated with each user, theline capacity (i.e., the achievable bit rate of the line assumingcontinues transmission) and the service rate (i.e., the bit rate thatthe customer is subscribed to or that the service provider is committedto provide). These two parameters may be different one with respect tothe other. Each customer may be subscribed to a different service rate.Each of the different lines connected to the Distribution Point may alsohave different capacities due to the differences in the distance fromthe DP, differences in the home wiring, differences in the linesattenuation and cross-talk within the binder (common in the highfrequencies) and differences in the background noise levels. Thedifferences in the line capacities and the service rates translate tovariations in the required transmission durations for each line. Thetransmission duty-cycle is determined according to the ratio between theservice bit-rate and the line capacity. For example, if the servicebit-rate is 100 Mbps and the line capacity is 200 Mbps we will have totransmit for approximately 50% of the time (i.e., ignoring the gaps andoverheads). In general, transmission duty-cycles may vary between say10% (1 Gbps line capacity, 100 Mbps service) to 100% (100 Mbps linecapacity, 100 Mbps service).

U.S. Pat. No. 7,817,745 to Cioffi et al entitled “Tonal Precoding”describes therein, a Digital Subscriber Line (DSL) communication systemwhich employs Discrete Multi-Tone (DMT) transmission and precoding inwhich U transmitters of U users are connected with U receivers employingFrequency Division Duplexing (FDD). The channel from the U transmittersto their U receivers can be modeled by a matrix channel H, whose sizeusually is U×U. The channel H can be decomposed into H=RQ using RQfactorization of a square matrix where the Q matrix can be used as alinear filter and R matrix can be used as a feedback filter foralleviating cross-talk. In one embodiment directed to by Cioffi et al, aG matrix in R=SG (where S is a diagonal scaling matrix that forces thediagonal elements of the triangular G to be all ones) can be interpretedas a set of precoder coefficients for the U users. These precodercoefficients can vary with each tone used by each user and depend uponthe encoding order of users selected for each tone. In practice, thechannel H is variable and the R and Q matrices are updated to adapt tosuch variability.

One of the embodiments directed to by Cioffi et al, employs adaptiveprecoding. Adaptive precoding adapts the precoding elements (e.g., theR, Q matrices, precoding coefficients, etc.) to changing channel ornoise conditions or to both. In the adaptive system directed to byCioffi et al, either the matrix R or the matrix Q or both may be updatedby a controller as frequently as needed to match the time-variations ofthe channel, as well as the noise. Such updating may be triggereddirectly (e.g., by changes to the channel matrix H or to the noisespatial correlation R_(nn)), or indirectly (e.g., by changes to the bittables or to the gain tables of the users of the vectored DSL system, orby changes to the precoding order within a tone, for the users of thevectored DSL system). The precoder may be updated after one or more ofthe receivers request new settings for the bit and gain tables of thevectored DSL transmitters. The precoder may also be updated after one ormore of the users of the vectored DSL system are turned off, or afterone or more users are added to the vectored DSL system.

U.S. Pat. No. 7,058,833 to Bremer et al, entitled “System and Method forMinimized Power Consumption for Frame and Cell Data TransmissionSystems” directs to a transmitter power manager for reducing power in acommunication system in which a communication device which is coupledwith a plurality of communication connections. The transmitter powermanager, which resides in the transmitter located in the central office,employs a data detector and power enabling circuitry. The data detectordetects activity corresponding to an incoming communication signal whichis to be transmitted. When such activity is detected the data detectorprovides a control signal to power enabling circuitry. The powerenabling circuitry is coupled to selected elements in the transmitterand with the transmitter signal generating circuitry which may bepowered down during periods of inactivity. When a control signal isreceived, which indicates the presence of an incoming communicationsignal which is to be transmitted, the power enabling circuitry providespower to the selected elements so that the selected elements are fullypowered and ready to transmit the communication signal. When thecommunication signal has been transmitted, the data detector detects theend of the transmission and provides a control signal to the powerenabling circuitry such that the selected components are then powereddown. That is, during periods of transmitter inactivity, selectedelements residing in the transmitter or the transmitter signalgenerating circuitry are powered down. When incoming communicationsignal, which are to be transmitted are detected, the power enablingcircuitry turns on the selected elements.

SUMMARY OF THE PRESENT DISCLOSED TECHNIQUE

It is an object of the disclosed technique to provide a noveltransmission scheme method and system in a multi-user TDD communicationssystem.

In accordance with the disclosed technique, there is thus provided atransmission scheme for a plurality of transceiver pairs. Eachtransceiver pair includes a first transceiver and a second transceiver.Each transceiver includes a transmitter and a receiver. Each of thetransceiver pairs define a downlink between the transmitter of a firsttransceiver and the receiver of a second transceiver and further definean uplink between the transmitter of the second transceiver and thereceiver of the first transceiver. The transmitter of the firsttransceiver includes a pre-precoder coupled with the input of a precoderand a transmitting port coupled with the output of the precoder and withthe receiver of the second transceiver. Each transceiver pair transmitsdata over at least a portion of a data transmission time-frame. The datatransmission frame is partitioned into a downlink transmission portionand an uplink transmission portion. The transmission scheme includes apartition of at least the downlink transmission portion of thetransmission frame into a plurality of precoding intervals. Eachprecoding interval is associated with a respective active group oftransceiver pairs. Each active group of transceiver pairs includes aplurality of transceiver pair. Each precoding interval terminating noearlier than the end of a respective downlink data transmission periodassociated with at least one of the transceiver pairs in the respectivetransceiver active group. A different respective precoder is associatedwith each the active groups. At least the transmitting ports of thefirst transceivers, which are not members of an active group, turn off.

In accordance with another aspect of the disclosed technique, there isthus provided a method for determining a transmission scheme for aplurality of transceiver pairs. Each transceiver pair includes a firsttransceiver and a second transceiver. Each transceiver includes atransmitter and a receiver. Each the transceiver pairs defines adownlink between the transmitter of a first transceiver and the receiverof a second transceiver and further define an uplink between thetransmitter of the second transceiver and the receiver of the firsttransceiver. Each transceiver pair transmits data over at least aportion of a data transmission frame. The data transmission frame ispartitioned into a downlink transmission portion and an uplinktransmission portion. The method includes the procedure of determiningat least one transmission mode. Each of the at least one transmissionmode is defined by a respective at least one active group of transceiverpairs at least during the downlink transmission portion. The methodfurther includes the procedure of estimating for each determinedtransmission mode, at least one expected respective bit-rate and atleast one expected respective power dissipation both associated with theat least one active group. The method also includes the procedure ofdetermining a transmission plan corresponding to each transmission modeaccording to data transmission information, power dissipationinformation, the at least one expected respective power dissipation, theat least one expected respective bit rate associate with the at leastone active group and according to service level agreement.

In accordance with a further aspect of the disclosed technique, there isthus provided a communications system. The communication system includesa distribution point. The distribution point includes a plurality ofdistribution point transceivers and a vectoring controller. Eachdistribution point transceiver includes respective pre-precoder, arespective transmission port and a respective receiver. The systemfurther includes a vectoring controller coupled with each of thedistribution point transceivers. Each distribution point transceivertransmits data over a data transmission frame. The data frame ispartitioned into an uplink transmission portion and a downlinktransmission portion. The vectoring controller at least determinesprecoding matrices corresponding to each of at least one active group ofdistribution point transmitters. At least the transmitting ports of thedistribution point transceivers, which are not members of an activegroup, turn off.

In accordance with another aspect of the disclosed technique, there isthus provided a transmission scheme for a plurality of transceiverpairs. Each transceiver pair includes a first transceiver and a secondtransceiver. Each transceiver includes a transmitter and a receiver.Each of the transceiver pairs define a downlink between the transmitterof a first transceiver and the receiver of a second transceiver andfurther define an uplink between the transmitter of the secondtransceiver and the receiver of the first transceiver. Each transceiverpair transmits data over at least a portion of a data transmissionframe. The data transmission frame is partitioned into a downlinktransmission portion and an uplink transmission portion. Thetransmission scheme includes a partition of at least the uplinktransmission portion of the data transmission frame into a plurality oftime segments. Each time segment is associated with a respective uplinkactive group of transceiver pairs. Each uplink active group oftransceiver pairs includes a plurality of transceiver pairs. Each timesegment terminates no earlier than the end of a respective uplink datatransmission period associated with at least one of the transceiverpairs in the respective transceiver active group. Cross-talkcancellation is performed only between the receivers of the firsttransceivers in the active group of transceiver pairs. The receivers ofthe first transmitters which are not members of an active group turnoff.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosed technique will be understood and appreciated more fullyfrom the following detailed description taken in conjunction with thedrawings in which:

FIG. 1 is a schematic illustration of a typical “last mile” connection,which is known in the art;

FIG. 2 is a schematic of a super-frame, which is known in the art;

FIG. 3 is a schematic illustration a simplified multi-usercommunications system in accordance with an embodiment of the disclosedtechnique;

FIG. 4 is a schematic illustration of a chart depicting the implicationsof turning-off one link in a vectored transmission group withoutcoordination in accordance with another embodiment the disclosedtechnique;

FIG. 5 is a schematic illustration of a transmission schemecorresponding to an exemplary transmission mode of the discontinuoustype, in accordance with another embodiment of the disclosed technique;

FIG. 6 is a schematic illustration of a transmission schemescorresponding to an exemplary transmission mode of to the continuoustype, accordance with a further embodiment of the disclosed technique;

FIG. 7 is a schematic illustration of a transmission scheme tocorresponding to an exemplary transmission mode corresponding to thehybrid type, in a communication system, in accordance with a furtherembodiment of the disclosed technique;

FIG. 8 is a schematic illustration of a transmission schemecorresponding to another exemplary transmission mode corresponding tothe hybrid type, in accordance with another embodiment of the disclosedtechnique;

FIG. 9 which is a schematic illustration of a vectored transmissionscheme in a multi-user TDD communications system, in accordance with afurther embodiment of the disclosed technique;

FIG. 10 is a schematic illustration of a graph, which depicts thetransmit Power Spectral Density (PSD) versus transmission frequency, forvarious precoders, in accordance with another embodiment of thedisclosed technique;

FIG. 11 is a schematic illustration of a communication system,constructed and operative in accordance with a further embodiment of thedisclosed technique;

FIG. 12 is a schematic illustration of a method for vectoredtransmission in accordance with another embodiment of the disclosedtechnique; and

FIG. 13 is a schematic illustration of a graph, depicting a graphicrepresentation of equation (7) for different number of links, inaccordance with a further embodiment of the disclosed technique.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The disclosed technique overcomes the disadvantages of the prior art byproviding a transmission scheme in a multi-user TDD communicationssystem, in which a transmission plan corresponds to one a transmissionmode. The transmission mode may of a discontinuous type, continuous typeor hybrid type. The discontinuous type optimizes either powerconsumption, the continuous type optimizes performance and the hybridtype facilitates a tradeoff between power consumption, performance andcomplexity respectively. The term ‘performance’ herein relates either tothe transmitted bit-rate (i.e., actual or maximum transmitted bit-rate)or to the channel capacity or to both. The system according to thedisclosed technique may operate according to transmission mode which isone of the above mentioned types continuously, or switch betweendifferent transmission modes from time to time (e.g., every frame, everyseveral frames, every super-frame or every several super-frames) asfurther explained below. Switching between the transmission modes may bebased on power and data traffic demands. For example, the systemaccording to the disclosed technique may determine on a frame by framebasis or on a super-frame by super-frame basis, according to whichtransmission mode a transmission plan shall be determined (i.e., thetransmission mode may not change at all). It is noted that the disclosedtechnique is applicable for both wired and wireless communicationsystems and thus the term ‘coupled’ herein below refers to wirelesslycoupled as well as to coupled by wire.

According to the transmission scheme of the disclosed technique, atransmission mode is defined according to the active group or groups oftransceivers. In a transmission mode corresponding to the discontinuousor hybrid transmission types, each transmission frame (i.e., either thedownlink transmission or the uplink transmission) is partitioned into aplurality of time intervals or time segments. In the downlink, the timesegments are also referred to as precoding intervals. The active groupof transceivers in each time segment (i.e., the group of transceiverstransmitting in a given time segment) may be different for each timesegment. According to the disclosed technique, a different precoder isemployed for each time segment or time segments according to the activegroup of transceivers transmitting during that time segment. The activegroups are selected to either minimize the power consumption of the DPand the CPE transmitters, or to facilitate a tradeoff between the powerconsumption, bit-rate and complexity. The term ‘complexity’ hereinrelates to the need to determine in real time or store in advance aprecoder matrix or matrices (i.e., as farther explained below) for eachactive group, given the memory allocated for storing the precodermatrices may be limited. In a transmission mode of the continuous type,there is one active group and all the transmitters continue to transmita residual cancellation signal, after transmitting the respective datathereof, until the transmitter with the longest data transmissionperiod, finishes transmitting the respective data thereof. Transceivers(i.e., pre-precoder and transmitting ports), which are not a part of theactive group may be turned off.

A communications system according to one embodiment of the disclosedtechnique (e.g., system 300 in FIG. 11 below) employs DiscreteMulti-Tone (DMT) transmission in which the transmission bandwidth isdivided into a plurality of sub-carriers. Due to the variation of thechannel characteristics over frequency, each sub-carrier is furtherassociated with a different respective precoder matrix and with arespective modulation constellation such as 64 Quadrature AmplitudeModulation (QAM), 256 QAM and the like. The respective precoder matrixis determined according to the channel characteristics associated withthe respective sub-carrier. It is noted that herein below the term‘precoder’ relates in unison the precoding matrices of all thesub-carriers. The respective modulation constellation of eachsub-carrier is typically determined according to the available signal tonoise ratio for the respective sub-carrier and additional parameterssuch as the required bit-rate and power constraints, error eventsstatistics and the like. The allocation of the constellations to thedifferent sub-carriers (i.e., the number of bits assigned to each symbolof each sub-carrier) is specified in a Bit Loading Table (BLT) for eachlink.

The time duration of a symbol (i.e., a time-slot) in a frame, along witha sub-carrier, define a Logical Allocation Unit (LAU). Thus, each framemay be regarded as a two dimensional array of LAUs. Each user isallocated respective set of LAUs that may be used for transmission(i.e., downlink transmission and uplink transmission). This set of LAUsdefines an ‘active-set’ used for transmission. Furthermore, the term“transmission opportunity” herein refers to either a time-slot or an LAUin an active-set. Furthermore, the term “transmission plan” refersherein to the allocation of transmission opportunities (i.e., eitherdownlink or uplink) for the DP transceivers and the CPEs transceivers.These transmission opportunities define the transmission duration ofeach transceiver. However, it is noted that the DP or the CPEs need notnecessarily use all the allocated transmission opportunities thereto.

In general, as mentioned above, in a transmission scheme according tothe disclosed technique, a transmission plan corresponds to atransmission mode. The transmission mode may be one of continuous type,discontinuous type or hybrid type as described herein below conjunctionwith FIGS. 5, 6, 7, 8 and 9. The active groups define the transmissionmode. The transmission plan (i.e., the allocation of transmissionopportunities) defines demarcation points (i.e., the points in time atwhich selected transmitters shall turn-off), which define the partitionof the data frame into time-segments and thus define the active group oftransmitters associated with each time segment. Each time segmentterminates no earlier than the end of a respective downlink datatransmission period of at least one of the transceivers the respectiveactive group. In the downlink, each time segment (i.e., each activegroup of transmitters) is associated with a respective precoder and arespective BLT for each link. Alternatively, several of the time segmentor the entire frame may be associated with the same BLT. In the uplink,each time segment in a transmission mode is associated with a respectiveBLT for each link. Furthermore, each active group in a transmission modeis associated with a respective expected bit-rate for each link and theexpected respective power dissipation for each one of the uplink and thedownlink.

Cancellation of cross-talk in general and of Far End Cross-talk (FEXT)in particular, involves pre-compensating (i.e., precoding) thetransmitted signal such that the cross-talk is negated at the receiver.Cancellation of cross-talk entails evaluating the effect of cross-talkthat is experienced on a known training signal during its transmissionthrough the communication channel. To apply appropriate precoding andthus substantially eliminate or minimize FEXT, the joint characteristicsof the various communication links (i.e., “channel characteristics”)need to be evaluated. This is also known as channel estimation. Channelestimation (or channel matrix estimation) involves estimating theelements in a channel matrix H as well as other performance parameterssuch as SNR and signal attenuation level. The channel matrix H isestimated for each sub-carrier k (denoted herein by H^((k))) for Mcommunication links, may be generally be represented by as follows:

$\begin{matrix}{H^{(k)} = {\begin{pmatrix}h_{11}^{(k)} & \ldots & h_{1M}^{(k)} \\\vdots & \ddots & \vdots \\h_{M\; 1}^{(k)} & \ldots & h_{MM}^{(k)}\end{pmatrix}.}} & (1)\end{matrix}$

The diagonal elements in channel matrix H (i.e., H_(ii) where i is aninteger between 1 and M) represent the direct i-th communication linkcorresponding to the transfer function of the transmitted signal on thei-th communication link. Off-diagonal elements in channel matrix H(i.e., H_(ij) where i≠j and are both integers between 1 and M) representthe FEXT coupling coefficients between the i-th and j-th communicationlinks.

In general, a received signal which was transmitted over a channelexhibiting cross-talk may be modeled as follows:

$\begin{matrix}{\begin{bmatrix}y_{k}^{1} \\\vdots \\y_{k}^{M}\end{bmatrix} = {{\begin{bmatrix}h_{11}^{(k)} & \ldots & h_{1M}^{(k)} \\\vdots & \ddots & \vdots \\h_{M\; 1}^{(k)} & \ldots & h_{MM}^{(k)}\end{bmatrix}\begin{bmatrix}x_{k}^{1} \\\vdots \\x_{k}^{M}\end{bmatrix}} + \begin{bmatrix}z_{k}^{1} \\\vdots \\z_{k}^{M}\end{bmatrix}}} & (1)\end{matrix}$

where y_(k) ^(m) is the received signal of the m^(th) user over thek^(th) sub-carrier, x_(k) ^(m) is the transmitted signal of the m^(th)user over the k^(th) sub-carrier and z_(k) ^(m) is a noise signal addedto x_(k) ^(m). In matrix notation equation (1) may be written asfollows:

y _(k) =H _(k) x _(k) +z _(k)  (2)

To alleviate the cross-talk, the transmitted signal is multiplied by arespective precoder matrix P as follows:

y _(k) =H _(k) P _(k) x _(k) +z _(k)  (3)

According to equation (3), the precoder output relating to the i^(th)communication link includes data relating to all the other links (i.e.,to enable the cancellation of the cross-talk from the other links). Thecross-talk may be alleviated by setting P_(k) to be equal to H_(k) ⁻¹.However, in such a case, the precoding gain is lost (i.e., since thediagonal elements of H_(k) are set to be equal to one). To maintain theprecoding gain, P_(k) may be set equal to the following:

P _(k) =H _(k) ⁻¹diag{H _(k)}  (4)

where diag{H_(k)} is a matrix including the diagonal elements of H_(k)with the off-diagonal elements set to zero.

The precoding described above in conjunction with equation (3) is knownas linear precoding. Nevertheless, according to the disclosed technique,non-linear precoding may alternatively be employed. Non-linear precodingattempts to cancel interferences known to the transmitter prior to thetransmission of the data. Thus the SNR may be increased withoutincreasing the transit power. Non-linear precoding, also referred to asdirty paper coding (DPC), includes methods such as Tomlinson-Harashimaprecoding (i.e., employing modulo arithmetic operation), Costaprecoding, the “vector perturbation technique”, and the like. Typically,non-linear precoder has better error related performance than a linearprecoder but a non-linear precoder is harder to implement.

In a vectored transmission system, the transmission bit-rate may beaffected by the cross-talk channels. In some cases, the cross-talk mayincrease the Signal to Noise Ratio (SNR) for a specific link andconsequently improve the achievable bit-rate for that link (e.g., whenthe attenuation in the cross-talk channel is lower than the attenuationin the direct channel). Thus, eliminating that cross-talk channel (e.g.,by turning off the interfering transmitter) may result in thedegradation of the transmitted bit-rate. Reference is now made to FIG.3, which is a schematic illustration a simplified multi-usercommunications system, generally referenced 100, in accordance with anembodiment of the disclosed technique. Exemplary system 100 includes twousers and exemplifies a mode where the attenuation in the cross-talkchannel is lower than the attenuation in the direct channel. System 100includes a first transceiver 101 ₁, second transceiver 101 ₂, a precoder104, a first receiver 108 ₁ and a second receiver 108 ₂. Each one offirst transceiver 101 ₁ and second transceiver 101 ₂ includes arespective first pre-precoder 102 ₁ and second pre-precoder 102 ₂. Eachone of first transceiver 101 ₁ and second transceiver 101 ₂ furtherinclude a respective first transmitting port 106 ₁ and secondtransmitting port 106 ₂.

Precoder 104 is coupled with first pre-precoder 102 ₁, secondpre-precoder 102 ₂, with first transmitting port 106 ₁ and with secondtransmitting port 102 ₂. First transmitting port 106 ₁ is coupled withfirst receiver 108 ₁ via a first direct link indicated by line 110 ₁.Second transmitting port 106 ₂ is coupled with second receiver 108 ₂ viaa second direct link indicated by line 110 ₂. First transmitting port106 ₁ is further coupled with second receiver 108 ₂ via a cross-talklink as indicated by dashed line 112.

First pre-precoder 102 ₁ and second pre-precoder 102 ₂ generate a streamof symbols to be transmitted to receivers 108 ₁ and 108 ₂ respectively.Precoder 104 encodes the symbols to overcome the interference caused bythe cross-talk between first direct link 110 ₁ and second direct link110 ₂. To that end, for example, linear precoding such as describedabove in equation (3) is employed. Thus, each transmitting porttransmits a signal which is a combination of the information related toall the transmitters. Due to the cross-talk between the channels, areceiver may receive the data directed thereto via the cross-talk linkas well as via the direct link. In the exemplary system depicted in FIG.3, receiver 108 ₂ receiver data transmitted thereto by transmitter 102 ₂via first transmitting port 106 ₁ as well as via transmitting port 106₂. Furthermore, the attenuation in both link 110 ₁ and link 110 ₂ is 20decibels (dB). The attenuation via cross-talk link 112 is only 10 dB.Thus, second transmitter 102 ₂ may communicate with second receiver 108₂ via first transmitting port 106 ₁ as well as via first transmittingport 106 ₁. In fact, such an optimal precoder may use both the directchannel and the cross-talk channel to communicate with receiver 108 ₂.

In light of the above, turning off either transmitter 102 ₁ ortransmitting port 106 ₁ shall result in a degradation in the performanceof the link between transmitter 102 ₂ and receiver 108 ₂ (i.e., sincethe precoder matrix was determined to facilitated overcoming cross-talkinterference when transmitting port 106 ₁ transmits data). Furthermore,when turning off transmitting port 106 ₁, the received signal power atreceiver 108 ₂ shall drop by 10 dB, which may result in errors since thesignal to noise ratio may now be too low for the required bit-rate andthe frequency equalizer (FEQ) at the receiver to will not match the newchannel conditions. Consequently, once transmitting port 106 ₁ is turnedoff, the transmit power from transmitting port 106 ₂ should be increasedby 10 dB. However, increasing the transmit power may not be possible dueto limitations on the upper limit of the transmitter power spectraldensity (PSD). In light of the above, for high cross-talk levelenvironments, power savings, achieved by turning off transmitter ports,may be traded off with achieving the highest possible bit-rate. It isnoted that this trade off does not relate to the ability to determinethe optimal precoder but rather to the theoretical achievable bit-ratewhen links are turned off.

As described above, shutting down a transmitter may result indegradation in performance. Reference is now further made to FIG. 4,which is a schematic illustration of a chart depicting the implicationsof turning-off one link (i.e., turning off at least the transmittingport) in a vectored transmission group without coordination (i.e.,keeping the same precoder), in accordance with another embodiment thedisclosed technique. FIG. 4 depicts the maximum bit-rate performance,designated ‘PHY-RATE [bps]’, for each link of a ten wire binder, whenall links are active and when one of the ports is disabled, withoutchanging the precoder matrix (i.e., no coordination). The resultsdepicted in FIG. 4 assume Zero Forcing (ZF) precoder employed with aBritish Telecom's (BT) ten line binder and a bandwidth of 100 MHz. Ascan be seen in FIG. 4, turning off a transmitting port results in thedegradation of the achievable bit-rate. To avoid such a degradation,turning off a port should be coordinated (i.e., a new precoder should bedetermined). To allow different transmission durations per link, asdescribed below, a new precoder should be determined according to thenumber of active links.

As mentioned above, in a transmission scheme according to the disclosedtechnique a transmission mode may be optimized for minimizing the powerconsumption of the transmitters. A transmission mode minimizing thepower consumption of the transmitters is of a discontinuous type.Reference is now made to FIG. 5, which is a schematic illustration of atransmission scheme corresponding to an exemplary transmission mode ofthe discontinuous type, generally referenced 120, in accordance withanother embodiment of the disclosed technique. FIG. 5 depicts a TDDframe 122, partitioned into a downlink time-period 124 and an uplinktime-period 126 in which M users are transmitting over M links and wherethe data transmission durations are different for each user. In FIG. 5,the different line capacities, the different service rates as well asthe different amount of data to be transmitted in each link may resultin different data transmission durations for each link. Further in FIG.5, the links are ordered according to their data transmission durations.The data transmission is referred to as payload in FIG. 5.

To minimizing the power consumption each transmitter turns off aftercompleting the transmission of the data thereof. To enable a transmitterto turn off after that transmitter transmitted the data thereof, adifferent precoder is determined for each group of active transceiver(i.e., the transmitters which currently transmit). In other words, aprecoder is determined for each time one of the transceivers is turnedoff and that precoder is employed by the remaining active transmitters.Accordingly, in FIG. 5, M−1 precoders are determined for M−1 differentactive group (i.e., a precoder is not needed when only one link isactive) and respective of M−1 precoding intervals (i.e., time segments).The term ‘active group’ relates herein above and below to the group oftransmitters currently transmitting (i.e., either the respective payloaddata thereof or a residual cancellation signal). For example, duringprecoding interval 128 ₁, the active group includes all M transmitters.During precoding interval 128 ₂, the active group includes transmitter 2to M. During precoding interval 128 ₃, the active group includestransmitter 3 to M and during precoding interval 128 _(M-1), the activegroup includes transmitter M−1 and M. Transmission scheme 120 presentedin conjunction with FIG. 5, requires memory allocation for storing allthe determined precoding matrices for each active group of transmitters.Alternatively, according to transmission scheme 120 presented inconjunction with FIG. 5, precoder corresponding to an active group isdetermined during the precoding intervals corresponding to previousactive groups or in real time when needed. For example, the precoder #3is determined during precoding intervals 128 ₁ and 128 ₂. In both case,the precoder changes whenever a transmitter (i.e., at least thetransmitting port) is turned off.

As mentioned above, in a transmission scheme according to the disclosedtechnique a transmission mode may be optimized to maximize the bit-rate.A transmission mode maximizing the bit-rate corresponds to thecontinuous type. Reference is now made to FIG. 6, which is a schematicillustration of a transmission schemes corresponding to an exemplarytransmission mode of to the continuous type, generally referenced 150,accordance with a further embodiment of the disclosed technique.Transmission scheme 150 depicted in FIG. 6 includes a TDD frame 152,partitioned into a downlink time-period 154 and an uplink-time period156, in which M users are transmitting over M links and where the datatransmission durations are different for each user. In FIG. 6, thedifferent line capacities, the different service rates as well as thedifferent amount of data to be transmitted in each link result indifferent data transmission durations for each link. Further in FIG. 6,the lines are ordered according to their data transmission durations.However, all the links in FIG. 6 continue to transmit residualcancellation signals after the end of the transmission of the respectivedata thereof. In other words, there is only one active group. Thus, ifone receiver receives the data directed thereto via a cross-talk channelas well as via the direct channel, that receiver shall continue toreceive the data directed thereto via the cross-talk channel even whenthe interfering transmitter finishes transmitting the data associatedtherewith and transmits only residual cancellation signals.Consequently, the SNR is not affected, the data bit-rate is maximizedand only a single precoder (i.e., a precoder matrix for eachsub-carrier) is determined. It is noted that that the transmitterstransmitting a residual cancellation signal do not have to generate anyresidual data. It is sufficient for the modulator in the respectivetransceiver to generate zeros at the input to the precoder matrix. Afterapplying the precoder, the signal at the output of the precoder may be anon-zero signal. It should be noted that for cases where the precoderexhibits only limited cross-talk cancellation capabilities, higher ratesmay actually be achieved for a small number of active links. This isbecause in such cases, the cross-talk is the dominant factor (i.e., asdescribed above in conjunction with FIG. 3) and turning off thetransmitters corresponding to these links eliminates the cross-talkinduced thereby.

In transmission scheme 150, all the downlink transmitters remain activeat least until the link with the longest data transmission duration hastransmitted the data thereof. Thus, transmission scheme 150 results inlarger power consumption relative to transmission scheme 120 in FIG. 5above. It is noted that the downlink receivers do not necessarily haveto be active when the corresponding transmitter transmits the residualdata thereof. The residual information does not include information thatneeds to be received by the receiver.

In a transmission scheme according to the disclosed technique, atransmission mode may facilitate a trade-off between the powerconsumption of the transmitters, the bit-rate and complexity. Attransmission mode facilitating such a trade-off corresponds to thehybrid type. Reference is now made to FIG. 7, which is a schematicillustration of a transmission scheme to corresponding to an exemplarytransmission mode corresponding to the hybrid type, generally referenced200, in a communication system, in accordance with a further embodimentof the disclosed technique. The transmission scheme depicted in FIG. 7exemplifies a trade-off between power consumption and transmissionbit-rate for M links. FIG. 7 depicts a TDD frame 202, partitioned into adownlink time-period 204 and an uplink-time period 206 in which M usersare transmitting over M links and where the transmission durations aredifferent for each user. Accordingly, the M links are ordered accordingto the transmission durations thereof.

In transmission scheme 200, the downlink time-period is partitioned intotwo precoding intervals 208 ₁ and 208 ₂, each associated with arespective active group of links. The first M−2 links are the firstgroup of active links associated with precoding interval 208 ₁ and ademarcation point is determined at the end of precoding interval 208 ₁(i.e., according to the link exhibiting the longest data transmissionduration of the first M−2 links). Link M−1 and link M are the secondgroup of active links associated with precoding interval 208 ₂. A firstprecoder is determined for the first group of active links duringprecoding interval 208 ₁ and a second precoder is determined for thesecond group of links during precoding interval 208 ₂. The dimensions ofthe first precoding matrix is MxM and the dimensions of the secondprecoding matrix is 2×2.

All the links transmit during precoding interval 208 ₁. Each link in thefirst group continues to transmit a residual cancellation signal, aftertransmitting the payload data thereof, until the end of precodinginterval 208 ₁ (i.e., the demarcation point) employing the firstprecoder. After the demarcation point, the first M−2 transmittersturn-off. Links M−1 and M continue to transmit the payload data thereofemploying the second precoder during precoding interval 208 ₂. Link M−1continues to transmit a residual cancellation signal after the end ofthe data transmission thereof until link M finishes transmitting thedata thereof. In the example set forth in FIG. 7, M−2 links seize theirtransmission before the end of the downlink transmission period, whileonly the other two links (i.e., link M−1 and link M) continuetransmitting, substantially during the entire downlink transmissionperiod. Turning off the transmitters (i.e., at least the transmittingports) before the last link finishes the data transmission thereofresults in substantial power savings. It is also noted that in theexample set forth in FIG. 7, link M−2 turns-off after precoding interval208 ₁ though there may still be data in the data queue associatedtherewith. In the example set forth in FIG. 7, the storage requirementsrequires storing only additional 4 parameters for each sub-carrier ofthe second precoder. According to another example, a precoder isdetermined for all M−1 links. The M−1 transceivers shut down after thelink with the second longest transmission period (i.e., link M−1 in FIG.7) ceases to transmit and only link M continues to transmit the datarespective thereof. In such a scheme only one precoder is determinedhowever power saving is still achieved by shutting down the transceiversprior to the end of the transmission of the link with the longesttransmission duration.

Reference is now made to FIG. 8, which is a schematic illustration of atransmission scheme corresponding to another exemplary transmission modecorresponding to the hybrid type, generally referenced 220, inaccordance with another embodiment of the disclosed technique. Thetransmission scheme depicted in FIG. 8 exemplifies a trade-off betweenpower consumption and transmission bit-rate for M links. FIG. 8 depictsa TDD frame 222, partitioned into a downlink time-period 224 and anuplink-time period 226 in which M users are transmitting over M linksand where the transmission durations are different for each user.Accordingly, the M links are ordered according to the transmissiondurations thereof.

In transmission scheme 220, the downlink time-period is partitioned intotwo precoding intervals 228 ₁ and 228 ₂, each associated with arespective active group of links. The first M−3 links are the firstgroup of active links associated with first precoding interval 228 ₁ anda demarcation point is determined at the end of precoding interval 228 ₁(i.e., according to the link exhibiting the longest data transmissionduration of the first M−3 links). Link M−2, link M−1 and link M are thesecond group of active links associated with second precoding interval228 ₂. A first precoder is determined for the first group of activelinks during precoding interval 228 ₁ and a second precoder isdetermined for the second group of active links during precodinginterval 228 ₂. The dimensions of the first precoding matrix is M×M andthe dimensions of the second precoding matrix is 3×3.

All the links transmit during precoding interval 228 ₁. The transceiversin the first active group continues to transmit a residual cancellationsignal after transmitting the payload data thereof, until the end oftime-period 228 ₁ (i.e., the demarcation point) employing the firstprecoder. After the demarcation point, the first M−3 transmittersturn-off. In transmission scheme 220, link M−2 is determined to be apart of the second active group of links even though the transmissionduration of the payload data thereof is shorter than precoding interval228 ₁. Thus, link M−2 continues to transmit residual cancelation signaland links M−1 and M continue to transmit the payload data thereofemploying the second precoder during the second preceding interval 228₂. Link M−1 continues to transmit a residual cancellation signal afterthe end of the data transmission thereof until link M finishestransmitting the data thereof. In general, in transmission modescorresponding to either the hybrid type or the continuous type, linksmay transmit a residual cancellation signal when the downlink datatransmission period does not overlap the precoding interval associatedwith the respective active group of links. For example, with referenceto FIG. 8, link 1 may first transmit residual cancellation signal andthen transmit the payload data during the respective precoding intervalthereof. As a further example, link 1 may start transmitting a residualcancellation signal, then transmit the payload data thereof, andcontinue to transmit the a residual cancellation signal after the end ofthe respective payload data transmission. In transmission scheme 220,M−3 links seize their transmission before the end of the downlinktransmission period, while only the other three links (i.e., link M−2,link M−1 and link M) continue transmitting, substantially during theentire downlink transmission period. Turning off the transmitters beforethe last link finishes the data transmission thereof results insubstantial power savings.

Reference is now made to FIG. 9, which is a schematic illustration of atransmission scheme in a multi-user TDD communications system, generallyreferenced 250, in accordance with a further embodiment of the disclosedtechnique. Transmission scheme 250 relates to operation in low powermode. FIG. 9 depicts two TDD frames 252 and 254. Each one of frames 252and 254 is, partitioned into a downlink time-period and an uplink-timeperiod. Frame 252 is partitioned into downlink time-period 256 anduplink-time period 258. Frame 254 is partitioned into downlinktime-period 260 and uplink-time period 262. In transmission scheme 250,M users are transmitting over M links and the transmission durations aredifferent for each user.

During downlink time-period 256 of frame 252, all the links transmitduring time-period 264 ₁. Each link in the first group continues totransmit a residual cancellation signal, after transmitting the payloaddata thereof, until the end of precoding interval 264 ₁ (i.e., thedemarcation point) employing a first precoder where the precodingmatrices are of size M×M. After the demarcation point, the first M−2transmitters turn-off. Links M−1 and M continue to transmit the datathereof employing a second precoder where the precoding matrices are ofsize 2×2. Link M−1 continues to transmit a residual cancellation signalafter the end of the data transmission thereof until Link M finishestransmitting the data thereof. During frame 254, transmitters #1 and #2have no data to transmit. Consequently, during precoding interval 266 ₁,transmitters #1 and #2 may turn off (i.e., turning off at least therespective transmitting ports thereof since greater power saving isachieved when a transmitting port is not active at all along aTDD-frame) and the M−2 other transmitters shall employ a precoder wherethe precoding matrices are of size M−2×M−2. After the demarcation pointat the end of precoding interval 266 ₁, the first M−4 transmitters turnoff. During precoding interval 266 ₂ Links M−1 and M continue totransmit the data thereof employing a second precoder where theprecoding matrices are of size 2×2. Consequently, power consumption isreduced without effecting performances.

In general, there are two low-power modes. The first mode is asynchronized mode and the second mode is the un-synchronized mode. Inthe synchronized mode, a transmitter that does not have data totransmit, does not turn-off completely, but rather continue to transmitsynchronization symbols which are used for determining the channelcharacteristics (e.g., frequency response, far and near end cross-talk)and consequently for determining the different precoder matrices.Consequently, when that transmitter is required to transmit data again,there is no need to re-acquire the channel but only to determine theprecoder. In this synchronized mode, the time period for transition toand from the low power state is on the order of tens of milliseconds. Inthe un-synchronized mode, a transmitter that has no data to transmitturns off completely and does not transmit anything. In this mode, whenthe transmitter is required to transmit data again, the channelcharacteristics need to be re-estimated before determining the precoder.Thus, the transition time to the active state is on the order of onesecond. However, the savings in power consumption is greater than in thesynchronous mode.

Reference is now made to FIG. 10, which is a schematic illustration of agraph, generally referenced 300, which depicts the transmit PowerSpectral Density (PSD) versus transmission frequency, for variousprecoders, in accordance with another embodiment of the disclosedtechnique. FIG. 10 depicts a plurality of plots of the transmit PSD forvarious precoder options where the lines indicate a “turned off” mode.The analysis relates to a Swisscom quad binder. Initially all the linesare active and calculated the appropriate precoder for this case. Thenone of the lines is turned off and precoder is recalculated with noapproximations. This new precoder is the optimal precoder and is alsotransparent to the customer premise equipment (CPE) receiver (i.e.,keeps the same channel gain).

As depicted in FIG. 10, once a transmitter is turned off, and assumingthe receivers keeps the same channel gains, the transmit power of othertransmitters may have to be increased. The result may be a violation ofPSD constraints, which in the example set forth in FIG. 10, may exceed,in some cases, 10 dB. Since the PSD constraints have to be compliedwith, the result is twofold. First Gain scaling has to be used at the DPtransmitter side to comply with the PSD constraints. Consequently, theuse of the new precoder is thus not transparent to the CPE, since thegain scaling factor has to be communicated to the CPE. Second, applyinggain scaling will reduce performance since for at least some of thesub-carriers the received signal to noise ratio will be lower, so asmaller constellation may be required (i.e., different bit-loading). Asdepicted in FIG. 10, turning off a transmitter in one link may impactthe performance of other links. In the some cases, such as depicted inFIG. 3 above, the attenuation in the cross-talk channel is lower thanthe attenuation direct channel. For example, with reference to FIG. 3,the direct link attenuation is 20 dB while the cross-talk channelattenuating is 10 dB.

As described above, a communications system according to the disclosedtechnique (e.g., system 300 in FIG. 11 below) employs DiscreteMulti-Tone (DMT) transmission in which the transmission bandwidth isdivided into a plurality of sub-carriers. Due to the variation of thechannel characteristics and noise over frequency, each sub-carrier isfurther associated with a respective modulation constellation such as 64Quadrature Amplitude Modulation (QAM), 256 QAM and the like. Therespective modulation constellation of each sub-channel is determinedaccording to the signal to noise ratio (SNR) of that sub-channel,required bit-rate, power constraints, CRC errors (i.e., error events)and the like. The allocation of the constellations to the differentsub-carriers (i.e., the number of bits assigned to each symbol of eachsub-carrier) is specified in a Bit Loading Table (BLT).

In general, a BLT is determined for each link and for each frame orsuper-frame. As mentioned above, turning off a transmitter changes thechannel characteristics and consequently the achievable bit-rate. Whenoperating according to a transmission mode of a discontinuous type orhybrid type, the system according to the disclosed technique maydetermine a single BLT, for each link, for the entire frame orsuper-frame based on the worst case conditions. However, determining aBLT for the entire frame or super-frame based on the worst caseconditions is not optimal in terms of the achievable bit-rate for thedetermined transmission mode. Thus, alternatively, the system accordingto the disclosed technique may determine a BLT for each link and foreach active group of transmitters, according to the channelcharacteristics associated with that active group. Once a transmitter isturned off, the channel characteristics over transmit frequency spectrummay change. Employing a previously determined BLT, after turning off atransmitter, may result in degradation in performance. Thus, to maintainoptimal performance, when a transmitter or transmitters are turned off,a new BLT may be required for remaining active transmitters since thechannel characteristics have changed.

Changing a precoder may result in a change in the precoding gains andconsequently in the overall gain (i.e., transmitter plus channel gains).Thus, when a precoder is changed, a gain level mismatch may occurbetween the gain parameters employed by the transmitter and gainparameters employed by the respective receiver. The change in theoverall gain may further result in the transmitted signal frequencyspectrum to violate a regulatory power spectral density (PSD) mask(e.g., PSD constraints, PSD values) or exceed above some upper limit(e.g., system design upper limit, component upper limit). The PSD maskdefines the maximum allowed transmit signal level per sub-carrier withinthe transmit frequency spectrum.

To prevent the PSD from exceeding some upper limit, the transmittedsignal frequency spectrum may need to be normalized. Since there are kprecoder matrices determined for each of the k sub-carrier, eachsub-carrier needs to be associated with a power normalization (scaling)factor. Furthermore, this scaling factor hast to be communicated to thereceiver to coordinate (e.g., equalize, normalize) the receiver gainswith the transmitter gains. To that end, the link between the receiverand the transmitter is used for conveying gain corrections by employing,for example, the Robust Management Channel (RMC) available in each TDDframe or a dedicated communication protocol or both.

Reference is now made to FIG. 11 which is a schematic illustration of acommunication system, generally referenced 300, constructed andoperative in accordance with a further embodiment of the disclosedtechnique. System 300 includes a distribution point (DP) 302 and aplurality of Customer Premise Equipment (CPEs) 304 ₁, 304 ₂, 304 ₃, . .. , 304 _(M). DP 302 includes a DP network interface 306, a DP dynamicresources allocator (DRA) 308, a DP controller 310, a DP vectoringcontroller 312 and a DP transceivers block 313. DP transceivers block313 includes a precoder 314 and a plurality of DP transceivers 316 ₁,316 ₂, 316 ₃, . . . , 316 _(M). Each one of transceivers 316 ₁, 316 ₂,316 ₃, . . . , 316 _(M) includes a respective pre-precoder 315 ₁, 315 ₂,315 ₃, . . . , 315 _(M), a respective transmitting port 317 ₁, 317 ₂,317 ₃, . . . , 317 _(M) and a respective receiver (not shown). DPdynamic resources allocator 308 includes a power controller 318. Eachone or CPEs 304 ₁, 304 ₂, 304 ₃, . . . , 304 _(M), such as CPE 304 ₁includes a respective CPE transceiver 320, a respective CPE networkaccess controller 322 and a respective CPE controller. Each one of DPtransceivers 316 ₁, 316 ₂, 316 ₃, . . . , 316 _(M) and the CPEtransceiver 320 of a respective one of CPEs 304 ₁, 304 ₂, 304 ₃, . . . ,304 _(M) includes a transmitter (not shown) and a receiver (also notshown).

Each one of transmitting ports 317 ₁, 317 ₂, 317 ₃, . . . , 317 _(M) iscoupled with a respective CPE transceiver 320 of a respective one of oneof CPEs 304 ₁, 304 ₂, 304 ₃, . . . , 304 _(M) defining transceiverpairs. The transmitting ports 317 ₁, 317 ₂, 317 ₃, . . . , 317 _(M) andthe respective receivers of each CPE transceiver 320 of a respective oneof CPEs 304 ₁, 304 ₂, 304 ₃, . . . , 304 _(M) coupled therewith, definea downlink. Furthermore, the receivers (not shown) of DP transceivers316 ₁, 316 ₂, 316 ₃, . . . , 316 _(M) and the respective transmitters ofeach CPE transceiver 320 of a respective one of CPEs 304 ₁, 304 ₂, 304₃, . . . , 304 _(M) coupled therewith, define an uplink. Accordingly,each one of DP transceivers 316 ₁, 316 ₂, 316 ₃, . . . , 316 _(M) formsa respective link 305 ₁, 305 ₂, 305 ₃, . . . , 305 _(M) with arespective CPE transceiver 320 of a respective one of one of CPEs 304 ₁,304 ₂, 304 ₃, . . . , 304 _(M). The term ‘link’ relates herein to twodevices communicating with each other (i.e., transmitting data to eachother and receiving data from each other).

DP Controller 310 is coupled with DP network access interface 306, DPdynamic resources allocator 308, DP vectoring controller 312 and witheach one of transceivers 316 ₁, 316 ₂, 316 ₃, . . . , 316 _(M) (depictedin FIG. 11 as coupling with transceiver block 313). DP Dynamic resourcesallocator 308 is further coupled with network access interface 306, DPvectoring controller and with each one of transceivers 316 ₁, 316 ₂, 316₃, . . . , 316 _(M) (also depicted in FIG. 11 as coupling withtransceiver block 313). DP Vectoring controller 312 is further coupledwith network access interface 306 and with each one of transceivers 316₁, 316 ₂, 316 ₃, . . . , 316 _(M) (also depicted in FIG. 11 as couplingwith transceiver block 313). Network access interface 306 is furthercoupled with a network (e.g., POTS or Internet—not shown). The inputs ofprecoder 314 are coupled with pre-precoders 315 ₁, 315 ₂, 315 ₃, . . . ,315 _(M). The outputs of precoder 314 are coupled with transmittingports 317 ₁, 317 ₂, 317 ₃, . . . , 317 _(M).

CPE controller 324 is coupled with CPE transceiver 320, CPE and withnetwork access interface 322. CPE network access interface 322 isfurther coupled with CPE transceiver 320 and with a customer premisenetwork (not shown). DP Network interface 306 provides each one of DPtransceivers 316 ₁, 316 ₂, 316 ₃, . . . , 316 _(M) the respectivedownstream data destined to the respective one of CPEs 304 ₁, 304 ₂, 304₃, . . . , 304 _(M). DP Network interface 306 further receives from eachone of DP transceivers 316 ₁, 316 ₂, 316 ₃, . . . , 316 _(M) theupstream data from the respective one of CPEs 304 ₁, 304 ₂, 304 ₃, . . ., 304 _(M). DP Network interface 306 may include buffers (not shown) fortemporarily storing downstream and upstream data when the rate of thedata received exceeds the rate of the transmitted data. DP networkinterface 306 further provides DP dynamic resources allocator 308 withdata queues status reports relating to the amount of data to be downlinktransmitted.

DP Controller 312 provides DP network interface 306, DP dynamicresources allocator 308, DP vectoring controller 312 and each one of DPtransceivers 316 ₁, 316 ₂, 316 ₃, . . . , 316 _(M) with extrinsicparameters relating to the operation of DP 302. DP Controller 312 mayreceive updates relating to these extrinsic parameters either from thenetwork via DP network interface 306 or via a user interface (notshown). The term ‘extrinsic parameters’ relates herein to parameterswhich are not determined by DP 302 and include information relating tothe configuration of thereof. For example, DP controller 310 providesdynamic resource allocator 308 with the maximum data rate or theguaranteed bit-rate specified in the service level agreement of eachuser associated with CPEs 304 ₁, 304 ₂, 304 ₃, . . . , 304 _(M). Dynamicresources allocator 308 may use this information when determining theallocated transmission opportunities for each one of CPEs 304 ₁, 304 ₂,304 ₃, . . . , 304 _(M) (e.g., the uplink or downlink time slots or thesub each allocated for each CPE). DP controller 310 may also providedynamic resources allocator 308 with the maximum allowed operatingtemperatures of DP 302 or of the components thereof or with ambienttemperature threshold and component temperature thresholds, to allowpower controller 318 to determine the required power dissipation ormaximum power dissipation of each one of transceivers 316 ₁, 316 ₂, 316₃, . . . , 316 _(M) so as to maintain the temperature below a determinedtemperature threshold.

DP Dynamic resources allocator 308 determines one or more active groupof transmitters. This active group or groups define transmission modes.The transmission mode or modes may correspond to the continuous type,the discontinuous type or the hybrid type as described above inconjunction with FIGS. 5, 6, 7, 8 and 9. DP Dynamic resources allocator308 provides the determined transmission mode or modes to DP vectoringcontroller 312. For each transmission mode, DP vectoring controller 312determines a precoder (i.e., the precoding matrices) corresponding toeach active group in the downlink and respective gain scaling factors.Furthermore, for each transmission mode, DP vectoring controller 312further determines at least one BLT for each active link. In general thenumber of BLTs may be equal or smaller than the number of active groups.In other words, for example, each active group may be associated with aBLT or all the active groups may be associated with the same BLT. As afurther alternative, several of the active groups may be associated withthe same BLT. DP vectoring controller 312 further determines theexpected bit-rate for each active group according to the determined BLTsand provides the expected respective bit-rate for each active group toDP dynamic resources allocator 308. DP vectoring controller 312determines the precoders gain scaling factors and BLTs according tobackchannel information. The backchannel information is furtherexplained below.

For each transmission mode, DP dynamic resources allocator determinesthe expected power dissipation. The expected power dissipation isrelated to the power dissipation of the transceivers (e.g., resultingfrom modulation parameters, transmission power and the like). For eachtransmission mode (i.e., combination of active groups), DP dynamicresource allocator 308 determines a transmission plan (i.e., theallocation of transmission opportunities). The transmission plan isdetermined according at least one data transmission information, powerdissipation information, the expected respective power dissipation, theexpected respective bit-rate associate with the active group or groupsand according to service level agreement. The transmission plan definesdemarcation points (i.e., the points in time at which selectedtransmitters shall turn-off) according to the active groups, whichdefine the partition of the data frame into time-segments. In otherwords, the transmission plan defines the transmission duration of eachtransceiver. Each time segment terminates no earlier than the end of arespective downlink data transmission period of at least one of thetransceivers the respective active group. DP dynamic resources allocator308 determines the transmission plan at least according to datatransmission information. Data transmission information (explainedfurther below) relates at least to the amount of data to be transmittedby each one of transceivers 316 ₁, 316 ₂, 316 ₃, . . . , 316 _(M) (alsoreferred to as ‘downlink data queues status’).

The above mentioned data transmission information relates at least todownlink data queue status. The data transmission information mayfurther include the amount of data to be transmitted in the uplink (alsoreferred to as ‘uplink data queues status’) that may conveyed to DP 302from CPEs 304 ₁, 304 ₂, 304 ₃, . . . , 304 _(M). The data transmissioninformation may further relate to the priority of the data to betransmitted and to guaranteed bit-rate specified in the contract of eachuser associated with CPEs 304 ₁, 304 ₂, 304 ₃, . . . , 304 _(M). Thedata transmission information may also relate to the capacity of eachone of links 305 ₁, 305 ₂, 305 ₃, . . . , 305 _(M). The datatransmission information may also include information relating topreviously used transmission opportunities by each of transceivers 316₁, 316 ₂, 316 ₃, . . . , 316 _(M) for the downlink and by each of CPEs304 ₁, 304 ₂, 304 ₃, . . . , 304 _(M) for the uplink, minimum amount ofdata each of transceivers 316 ₁, 316 ₂, 316 ₃, . . . , 316 _(M) and eachof CPEs 304 ₁, 304 ₂, 304 ₃, . . . , 304 _(M) is require to transmit(i.e., may be smaller or larger than the stored amount of data to betransmitted). The term ‘amount of data’ refers to herein to the numberof units of information (e.g., bits, bytes or symbols). DP dynamicresources allocator 308 determined the data transmission informationaccording to received, data queue status reports, bandwidth reports aswell as the extrinsic parameters from DP controller 310. The data queuestatus reports include information relating to the required amount ofdata to be transmitted by each one of transceivers 316 ₁, 316 ₂, 316 ₃,. . . , 316 _(M) in the downlink and by each of CPEs 304 ₁, 304 ₂, 304₃, . . . , 304 _(M) in the uplink. Each of CPEs 304 ₁, 304 ₂, 304 ₃, . .. , 304 _(M) may transmit the uplink data queues status thereof to DP302 periodically or upon request from DP dynamic resources allocator308. The bandwidth reports may include information relating to thepreviously used transmission opportunities by each of transceivers 316₁, 316 ₂, 316 ₃, . . . , 316 _(M) for the downlink, and by each of CPEs304 ₁, 304 ₂, 304 ₃, . . . , 304 _(M) for the uplink. The bandwidthreports may also include information relating to the minimum amount ofdata each of transceivers 316 ₁, 316 ₂, 316 ₃, . . . , 316 _(M) and eachof CPEs 304 ₁, 304 ₂, 304 ₃, . . . , 304 _(M) is require to transmit(i.e., may be smaller or larger than the stored amount of data to betransmitted).

The above mentioned backchannel information according to which DPvectoring controller 312 determines the precoder and gain scalingfactors for each transmission mode or modes includes informationreceived from each one of CPEs 304 ₁ 304 ₂, 304 ₃, . . . , 304 _(M),which relates to channel characteristics the transmission channel. Thesecharacteristics relate, for example, to the downlink Far End Cross-talk(FEXT) measurements, SNR and signal attenuation. The parameters may bedetermined directly by each one of CPEs 304 ₁ 304 ₂, 304 ₃, . . . , 304_(M) and communicated to DP 302, according to probing signalstransmitted by DP vectoring controller 312 via each one of transceivers316 ₁, 316 ₂, 316 ₃, . . . , 316 _(M) to the respective one of CPEs 304₁ 304 ₂, 304 ₃, . . . , 304 _(M). Alternatively, the above mentionchannel characteristics may be determined the DP 302, for example,according to information relating to errors, received from the CPE(which are determined by the CPE according to the received probingsignals). Since the probing signal transmitted by each one oftransceivers 316 ₁, 316 ₂, 316 ₃, . . . , 316 _(M) are known to CPEs 304₁ 304 ₂, 304 ₃, . . . , 304 _(M), each one of CPEs 304 ₁ 304 ₂, 304 ₃, .. . , 304 _(M) can evaluate the channel characteristics and transmitsthe determined characteristics to DP 302. DP vectoring controller 312employs the channel characteristics to determine the channel responsematrices (i.e., a matrix for each sub-channel). Consequently, DPvectoring controller 312 can determine the effect of turning offtransceivers 316 ₁, 316 ₂, 316 ₃, . . . , 316 _(M) as well as turningoff transceiver 120 CPEs 304 ₁ 304 ₂, 304 ₃, . . . , 304 _(M) of on thechannel response matrix. Thus, DP vectoring controller 312 can determinethe effect of turning off transceivers 316 ₁, 316 ₂, 316 ₃, . . . , 316_(M) on the precoder (i.e., on the precoder matrices).

The power dissipation information of DP 302 mentioned above includes atleast the maximum allowed power dissipation of DP 302. For example,dynamic resources allocator 308 receives information relating to theambient temperature of DP 302 or of components (e.g., chips) within DP302 from various sensors (not show) inside DP 302. Furthermore, DPdynamic resources allocator 308 may receive additional informationrelating to the actual power dissipation of DP 302 (e.g., from currentand voltage sensors—both not shown). The power dissipation informationreceived by dynamic resources allocator 308 may further includeinformation relating required operating temperatures of DP 302 or of thecomponents thereof or relating to the ambient temperature threshold andcomponent temperature thresholds. Alternatively or additionally, Dynamicresources allocator 308 may further receive from DP controller 310information relating to the allowed energy or power dissipation of DP302. Power controller 318 employs this information when selecting atransmission mode so as to meet either power dissipation requirements,temperature requirements or both. For example, these temperaturerequirements include maintaining the temperature of DP 302 below adetermined temperature threshold (i.e., either the temperature within DP302 or the temperature of components of DP 302). The power requirementsmay be, for example, a power dissipation threshold of DP 302.

When, for example, at least one of the links is required to transmitlarge amount of information with high priority, DP Dynamic resourcesallocator 308 may select a continuous mode transmission mode maximizingthe transmission bit-rate such as described above in conjunction withFIG. 6. As a further example, when all of the links are required totransmit low amounts of information with low priority, DP Dynamicresources allocator 308 may select a discontinuous mode, minimizing thetransmission power such as described above in conjunction with FIG. 5.According to another example, some of the links are required to transmitlarge amounts of information with low priority, DP Dynamic resourcesallocator 308 shall select a hybrid transmission mode which trades-offbetween transmission power and bit-rate such as described above inconjunction with FIG. 7.

Each one of DP transceivers 316 ₁, 316 ₂, 316 ₃, . . . , 316 _(M)transmits data to CPE transceiver 320 of the respective one of CPEs 304₁, 304 ₂, 304 ₃, . . . , 304 _(M). When transmitting downstream data,each one of pre-precoders 315 ₁, 315 ₂, 315 ₃, . . . , 315 _(M) performstransmission operation such as framing and Forward Error Correction(FEC). Precoder 314 multiples each data stream corresponding to eachsub-carrier respective of each transceiver 316 ₁, 316 ₂, 316 ₃, . . . ,316 _(M), by the respective precoding matrix corresponding to eachsub-carrier. Each one of transmitting ports 317 ₁, 317 ₂, 317 ₃, . . . ,317 _(M), modulates the respective output of precoder 314 by employingthe Inverse Fourier Transform (IFT) and transmits the modulated signalto the respective one of CPEs 304 ₁, 304 ₂, 304 ₃, . . . , 304 _(M). Itis noted that, for each DMT sub-carrier, precoding is performedaccording to the respective precoder matrix corresponding to the currentactive group of transceiver. When receiving upstream data, each one oftransceivers 316 ₁, 316 ₂, 316 ₃, . . . , 316 _(M) performs receptionoperations such as filtering, demodulation, cross-talk cancellation, FECdecoding. All transceivers 316 ₁, 316 ₂, 316 ₃, . . . , 316 _(M)simultaneously transmit data frames starting at the same time and overthe transmission bandwidth.

CPE Controller 324 provides CPE network interface 322, CPE transceivers320 with extrinsic parameters relating to the operation of CPE 304. DPController 312 may receive updates relating to these extrinsicparameters either from the network via CPE network interface 306 or viaa management interface (not shown). These extrinsic parameters are, forexample, the guaranteed and maximum uplink data rate of the respectiveCPE. CPE Network interface controller 322 receives from the user deviceor devices (e.g., computers, routers, smartphones—all not shown)upstream data and provides this upstream data to transceiver 320. CPENetwork interface 322 receives from transceiver 320 downstream data andprovides this downstream data to the user device or devices. CPE Networkinterface 306 may include buffers (not shown) for temporarily storingdownstream and upstream data when the rate of the data received exceedsthe rate of the transmitted data (i.e., either toward the DP or thenetwork). When transmitting upstream data, CPE transceiver 320 performstransmission operations such as framing, FEC, encoding and modulation.When receiving downstream data, transceiver 320 performs receptionoperations such as filtering, demodulation and FEC decoding.

According to another embodiment of the disclosed technique, and stillreferring to FIG. 11, DP vectoring controller 308 determines thebit-rate for each link in each one or more active groups, according toone or more BLT associated with the one or more active groups. DPdynamic resources allocator 308 determines a transmission plan accordingto at least one of the bit-rate for each link in each one or more activegroup (i.e., as determined by DP vectoring controller 308) and powerdissipation information, data transmission information, service levelagreement or any combination thereof. The transmission plan, in fact,comprises a combination of active groups, each one with the respectiveBLT per link thereof where the demarcation points between the activegroups are parameters to be optimized to facilitate the above discussedtradeoff between power and performance.

Reference is now made to FIG. 12 which is a schematic illustration of amethod for vectored transmission in accordance with another embodimentof the disclosed technique. In procedure 400, at least one transmissionmode is determined. The transmission mode is one of continuous type,discontinuous type or hybrid type as described above in conjunction withFIGS. 5, 6, 7, 8 and 9. Each of the at least one transmission mode isdefined by a respective active group or a combination of active groupsof transmitters. The transmission mode is determined for each frame orfor time to time as explained above. With reference to FIG. 11, DPdynamic resources allocator determines at least one transmission mode.

In procedure 402, a respective precoder and respective gain scalingfactors are determined for each active group of transceivers. Theprecoder includes a precoding matrix for each sub-carrier and the gainscaling factor is determined for each sub-carrier in the downlink. Withreference to FIG. 11, DP vectoring controller 312 determines arespective precoder and respective gain scaling factors for each activegroup of transmitters in each determined transmission mode.

In procedure 404, at least one BLT is determined for each determinedtransmission mode and for each active link. As mentioned above, BLTs isdetermined according to the channel characteristics. Since turning off atransmitter may change the resulting signal to noise ratio (SNR) for alink, a new BLT may need to be determined for that link once atransmitter is turned off. It is noted that the BLTs are determined forthe downlink as well as for the uplink (as further explained below).Furthermore, in general the number of BLTs may be equal or smaller fromthe number of active groups. In other words, each active group may beassociated with a BLT or all the active groups may be associated withthe same BLT. As a further alternative, several of the active groups maybe associated with the same BLT. With reference to FIG. 11, DP vectoringcontroller 312 determines a respective BLT for each active group oftransmitters in each determined transmission mode.

In procedure 406, for each determined transmission mode, at least oneexpected respective bit-rate and at least one expected respective powerdissipation are associated with the at least one active group areestimated. (i.e., the bit rate associated with each active group indefining the transmission mode). The expected bit-rates are determinedaccording to the determined respective BLT or BLTs. The expected powerdissipation relates to the power dissipation of the transceivers (e.g.,resulting from modulation parameters, transmission power and the like).With reference to FIG. 11, DP vectoring controller 312 estimates theexpected respective bit-rate and DP dynamic resources allocator 308estimates the expected respective power consumption for each determinedtransmission mode.

In procedure 408, a transmission plan corresponding to each transmissionmode is determined according to at least one of data transmissioninformation, power dissipation information, the at least one expectedrespective power dissipation, the at least one expected respective bitrate associate with the at least one active group and according toservice level agreement. The transmission plan defines the transmissionduration for each transceiver in each active group. The transmissionplan includes the allocation of transmission opportunities for thetransceivers. The transmission plan defines demarcation points (i.e.,the points in time at which selected transmitters shall turn-off), whichdefine the partition of the data frame into time-segments (i.e.,precoding intervals in the downlink). Each time segment terminates noearlier than the end of a respective downlink data transmission periodof at least one of the transceivers the respective active group. Withreference to FIG. 11, DP dynamic resource allocator 308 determines atransmission plan corresponding to the each determined transmissionmode.

In procedure 412, the determined transmission plan, BLTs and gainscaling factors corresponding to the selected transmission mode aretransmitted to the CPEs. Thus, the CPEs shall be able to demodulate andscale the received signal thereby. The BLT or BLTs and the gain scalingfactors are transmitted to the CPEs over a management channel. The CPEshall use these BLTs and gain scaling factors according to thedetermined transmission scheme. The management channel is a specialcommunication channel (i.e., either physical or logical) dedicated forthe transmission of communications control information (e.g., allocatedtransmission opportunities, BLTs, scaling factors, bandwidth reports andthe like) between the DP and the CPEs. With reference to Figure, 11 DPtransceivers 316 ₁, 316 ₂, 316 ₃, . . . , 316 _(M) transmit thedetermined BLT and gain scaling factors corresponding to the selectedtransmission mode to CPEs 304 ₁, 304 ₂, 304 ₃, . . . , 304 _(M).

Following is an example according to which a transmission mode may bedetermined. The following example assumes N links of equal link capacityor maximum specified bit-rate. As mentioned above, when maximizing thebit-rate all the links continue to transmit residual cancelation datauntil the link with the largest transmission duration ends thetransmission thereof. In such a case the DP power consumption may beestimated as follows:

Power_(continuous) =N·max(R ₁ ,R ₂ , . . . R _(N))/R _(max)  (5)

where R_(max) the maximum possible bit rate achieved when all links areactive and R₁, R₂, . . . R_(N) are the traffic demands for link.

When each transmitter turns off after transmitting the respective datathereof (i.e., minimizing power in the discontinuous mode), the DP powerconsumption may be estimated as follows:

Power_(discont)=Σ_(i=1) ^(N) R _(i) /R _(discont)  (6)

where R_(discont) is the available bit-rate when discontinuous operationis enabled.

Assuming that, for example, the traffic demands per frame of all thelinks are a fraction, a, of the traffic demand per frame of the linkwith the highest traffic demand, than, employing discontinuous modesaves power only when the following occurs:

$\begin{matrix}{\frac{R_{discont}}{R_{\max}} > \frac{\left\lbrack {1 + {\left( {N - 1} \right)\alpha}} \right\rbrack}{N}} & (7)\end{matrix}$

Reference is now made to FIG. 13 which is a schematic illustration of agraph, generally referenced 450, depicting a graphic representation ofequation (7) for different number of links, in accordance with a furtherembodiment of the disclosed technique. Line 452 is a graphicalrepresentation of equation (7) when two links are employed (i.e., N=2).Hatched double dotted line 454 is a graphical representation of equation(7) when four links are employed (i.e., N=4). Hatched line 456 is agraphical representation of equation (7) when eight links are employed(i.e., N=8). Dotted line 458 is a graphical representation of equation(7) when sixteen links are employed (i.e., N=16). In FIG. 13 the areaabove each of lines 452, 454, 456 and 458 represents the values of α and

$\frac{R_{discont}}{R_{\max}}$

for which discontinuous mode results in savings of power for eachrespective number of links. The area below each of lines 452, 454, 456and 458 represents the values of α and

$\frac{R_{discont}}{R_{\max}}$

for which discontinuous mode does not results in savings of power foreach respective number of links. Thus, it may be concluded thatdiscontinuous operation may results in saving power when there issubstantial difference in traffic demands between the links or whenthere is no substantial degradation in performance when transmitters areturned off. Furthermore, discontinuous operation may save power when alarge group of links are employed.

The description herein above relates to the downlink. However, thedisclosed technique may be applied to the uplink as well. Withreferenced to FIG. 11 and as mentioned above, DP dynamic resourcesallocator 308 receives bandwidth reports from network interface 306 andfrom CPEs 304 ₁, 304 ₂, 304 ₃, . . . , 304 _(M) which includeinformation relating to the required amount of data to be transmitted byeach one of transceivers 316 ₁, 316 ₂, 316 ₃, . . . 316 _(M) and by eachof CPEs 304 ₁, 304 ₂, 304 ₃, . . . , 304 _(M). Similar to the downlink,the transmission mode determined DP vectoring controller 312 defines theactive groups of uplink transmitters. The transmission plan definesuplink demarcation points (i.e., the points in time at which selectedtransmitters shall turn-off), which define the partition of the uplinkportion of the data frame in time-segments and thus, the active group oftransmitters associated with each time segment. Uplink transmitterswhich are not members of an active group may turn off. Consequently, theuplink receivers (i.e., the receivers located at DP 302) may also turnoff. DP vectoring controller 312 further determines a respective a BLTfor each one of CPEs 304 ₁, 304 ₂, 304 ₃, . . . , 304 _(M), for eachtime segment in each uplink transmission mode and consequently with theexpected bit-rates for each link. It is, however, noted that for theuplink there is no need to determine a respective precoder and thus noneed to determine gain scaling factors since the CPEs do not employprecoding. Rather DP 302 performs cross-talk cancellation. Therefore, DPvectoring controller determines respective cross-talk cancellationcoefficients for each active group. DP dynamic resources allocator 308allocates transmission opportunities to CPEs 304 ₁, 304 ₂, 304 ₃, . . ., 304 _(M) during the uplink transmission portion of the transmissionframe. DP dynamic resources allocator 308 transmits to CPEs 304 ₁, 304₂, 304 ₃, . . . , 304 _(M) the determined transmission mode andtransmission scheme (i.e., for the entire frame for both the downlinktransmission portion and the uplink transmission portion) over amanagement channel related to each frame or super-frame. When receivinguplink transmission from CPES 304 ₁, 304 ₂, 304 ₃, . . . , 304 _(M), DP302 may perform cross-talk cancellation only with the receiverscorresponding to the active group of CPEs, to facilitate power saving.The above mentioned management channel is preferably a separate physicalchannel (e.g., G.fast robust management channel—RMC), or logical channel(e.g., G.fast EOC) or a combination thereof. The management channelconveys communications control information (e.g., allocated transmissionopportunities, BLTs, scaling factors, bandwidth reports and the like)between the DP and the CPEs. Preferably the communication controlinformation is communicated in prior to the pertinent transmissionframe. This channel is located in both the downlink transmission portionand the uplink transmission portion.

It will be appreciated by persons skilled in the art that the disclosedtechnique is not limited to what has been particularly shown anddescribed hereinabove. Rather the scope of the disclosed technique isdefined only by the claims, which follow.

1. A transmission scheme for a plurality of transceiver pairs, each transceiver pair including a first transceiver and a second transceiver, each transceiver including a transmitter and a receiver, each said transceiver pairs defining a downlink between the transmitter of a first transceiver and the receiver of a second transceiver and further defining an uplink between the transmitter of said second transceiver and the receiver of said first transceiver, said transmitter of said first transceiver includes a pre-precoder coupled with the input of a precoder and a transmitting port coupled with the output of said precoder and with said receiver of said second transceiver, each transceiver pair transmitting data over at least a portion of a data transmission frame, said data transmission frame being partitioned into a downlink transmission portion and an uplink transmission portion, said transmission scheme including: a partition of at least said downlink transmission portion of said data transmission frame into a plurality of precoding intervals, each precoding interval being associated with a respective active group of transceiver pairs, each active group of transceiver pairs including a plurality of transceiver pairs, each precoding interval terminating no earlier than the end of a respective downlink data transmission period associated with at least one of the transceiver pairs in the respective transceiver active group, wherein, a different respective precoder is associated with each said active groups, wherein, at least the transmitting ports of the first transceivers, which are not members of an active group, turn off; and wherein each said transceiver pair transmitting a residual cancellation signal during said precoding interval when the duration of said respective precoding interval is longer than said respective downlink data transmission period and when said downlink data transmission period and said respective precoding interval do not overlap one with the other.
 2. (canceled)
 3. The transmission scheme according to claim 1, wherein a respective bit loading table is determined for each transceiver pair for at least the entire transmission frame.
 4. The transmission scheme according to claim 1, wherein a respective bit loading table is determined for each transceiver pair for each precoding interval.
 5. The transmission scheme according to claim 1, wherein respective gain scaling factors are determined for each different respective precoder.
 6. The transmission scheme according to claim 1, wherein a transmission plan corresponding to a transmission mode defines down link data transmission opportunities which further define said downlink transmission period, said transmission plan further defining uplink transmission opportunities which further define an uplink transmission period.
 7. The transmission scheme according to claim 6, further including a partition said uplink transmission portion of said data transmission frame into a plurality of time-segments, each time-segment being associated with a respective uplink active group of transceiver pairs, each uplink active group of transceiver pairs including a plurality of transceiver pairs, the uplink active groups corresponding to said transmission mode each time segment terminating no earlier than the end of said respective uplink data transmission period of at least one of the transceiver pairs in the respective transceiver active group, wherein cross-talk cancellation is performed only between the active group of transceivers, and wherein the transmitters of the second transceivers, which are not members of an active group, turn off.
 8. The transmission scheme according to claim 1, wherein each of said active groups is associated with respective expected bit-rate and respective expected power dissipation.
 9. The transmission scheme according to claim 8, wherein a transmission mode is selected according to the respective expected bit-rates and the respective expected power dissipations and according to information related to the data queues status and power dissipation information.
 10. The transmission scheme according to claim 9, wherein said power dissipation information at least include operating temperature and wherein said data transmission information at least include the required amount of data to be transmitted in said transmission frame. 11.-21. (canceled)
 22. A communications system comprising: a distribution point at least including: a plurality of distribution point transceivers, each of said distribution point transceivers including a respective pre-precoder, a respective transmission port and a respective receiver, each distribution point transceiver transmitting data over a data transmission frame, said data frame being partitioned into an uplink transmission portion and a downlink transmission portion; dynamic resources allocator, coupled with said distribution point transceivers and with said vectoring controller, said dynamic resources allocator determines at least one transmission mode, each one of said at least one transmission mode being defined by at least one respective active group of transceiver pairs, said dynamic resources allocator further determines a transmission plan for each of said at least transmission mode, said transmission plan defines the transmission duration for each transceiver pair in each active group, said dynamic resources allocator estimates expected power dissipation according to the transmission duration of each transceiver in each active group; and a vectoring controller, coupled with said distribution point transceivers, said vectoring controller at least determining precoding matrices corresponding to each of at least one active group of distribution point transmitters, said vectoring controller determines at least one bit loading table each of said at least one transmission mode, said vectoring controller further estimates expected bit rate associated with each said at least one transmission mode according to said at least one bit loading table; and wherein, at least the transmitting ports of the distribution point transceivers, which are not members of an active group, turn off. 23.-24. (canceled)
 25. The system according to claim 24, wherein for each said at least one transmission mode, said vectoring controller determines a respective bit loading table for each of said distribution point transceivers for the entire transmission frame.
 26. The system according to claim 24, wherein for each said at least one transmission mode said vectoring controller determines a respective bit loading table for each of said distribution point transceivers and for each active group.
 27. (canceled)
 28. The system according to claim 22, wherein said dynamic resources allocator selects one of said at least one transmission mode, according to at least one of said expected respective power dissipation and said expected bit rate and a respective at least one of data transmission information and power dissipation information.
 29. The system according to claim 22, wherein said transmission plan includes demarcation points, said demarcation points define a partition of at least said downlink portion of said data transmission frame into precoding intervals, each of said active groups is associated with a respective precoding interval, each precoding interval terminates no earlier than the end of a respective downlink data transmission period of at least one of the transceivers in the respective active group, the transmission plan being determined at least according information related to the data queues status and the selected transmission mode.
 30. The system according to claim 29, wherein each said distribution point transceivers transmit a residual cancellation signal during said precoding interval when the duration of said respective precoding interval is longer than the respective downlink data transmission period thereof and when said downlink data transmission period and said respective precoding interval do not overlap one with the other.
 31. The system according to claim 29, said transmission plan further includes the allocation of downlink and uplink transmission opportunities, and wherein said dynamic resources allocator determines said uplink transmission opportunities according to information related to uplink data queue status transmitted by said customer premise equipment transmitters to said distribution point.
 32. The system according to claim 29, further including at least one customer premise equipment, including a customer premise equipment transceiver, said customer premise equipment transceiver including a customer premise equipment transmitter and a customer premise equipment receiver, said customer premise equipment transmitter transmitting data over said data communication frame during at least a portion of said uplink transmission.
 33. The system according to claim 32, wherein said transmission mode further defining at least one active group of customer premise transmitters, said transmission plan further defines uplink demarcation points which define the partition said uplink transmission portion of said data transmission frame into time segments, each time segment is associated with a respective active group of customer premise equipment transceivers, each time segment terminating no earlier than the end of a respective uplink data transmission period of at least one of the customer premise equipment transceiver in the respective transceiver active group, wherein cross-talk cancellation is performed only between the active group of customer premise equipment transceivers, and wherein the receivers in said distribution point transceivers, corresponding to transmitters of the customer premise equipment transceivers which are not members of an active group, turn off.
 34. The system according to claim 32, wherein said customer premise equipment further includes a customer premise equipment controller, coupled with said customer premise equipment transceiver, said customer premise equipment provides said customer premise equipment transceivers with extrinsic parameters relating to the operation thereof, said extrinsic parameters at least related to the service level agreement specified for each user.
 35. The system according to claim 22, wherein for each transmission mode said vectoring controller determines respective gain scaling factors for each different precoding matrix corresponding to each of said at least one sub-carrier.
 36. The system according to claim 22, wherein said distribution point further includes a distribution point controller, coupled with each of said at least one distribution point transceiver, with said dynamic resources allocator and with said vectoring controller, said distribution point controller provides said distribution point transceivers and said dynamic resources allocator with extrinsic parameters relating to said distribution point, said extrinsic parameters at least include information related to the service level agreement specified for each user.
 37. The system according to claim 22, further includes a precoder coupled with each pre-precoder and with each transmitting port, said precoder precodes each data stream corresponding to each sub-carrier respective of each of said transceivers. 38.-40. (canceled) 